The Biosynthesis of Thyroxine: Incorporation of [U-14c]tyrosine Into Thyroglobulin by Mouse Thyroid Glands in Vivo and in Vitro

Abstract

The incorporation in vivo of [U-14C]tyrosine into 19S thyroglobulin by mice was achieved to a level of 2\m=.\85 c.p.m./\g=m\g. This level of incorporation was insufficient to permit the isolation of 14C-labelled iodinated tyrosines or residues. Isolated mouse thyroid lobes were used as an in-vitro system for the synthesis of 19S thyroglobulin. The lobes continued to incorporate 131I into 19S thyroglobulin for at least 48 h and this incorporation of iodine was specifically inhibited by propylthiouracil. The isolated mouse thyroid lobe in-vitro system was used to incorporate 14C-labelled amino acids into 19S thyroglobulin. [U-14C]Tyrosine was incorporated to a level of 1150 c.p.m./\g=m\g. Electrophoretic separations of enzymic hydrolysates of [14C]tyrosine-labelled 19S thyroglobulin showed the presence of [14C]tyrosine, [14C]monoiodotyrosine, [14C]di-iodotyrosine and [14C]thyroxine. The presence of [14C]tyrosine, [14C]monoiodotyrosine and [14C]thyroxine was demonstrated by chromatography of the eluates from the electrophoretic separations.The results provide evidence for the utilization of tyrosyl residues within the thyroglobulin molecule for iodination and subsequent coupling to form thyroxine.